Background: Fetal or neonatal alloimmune thrombocytopenia (FMAIT) results from a maternal alloimmunization against fetal platelet (PLT) antigens. In Caucasian persons, HPA-1a is the most frequently implicated antigen. During the past few years, FMAIT has been reported associated with rare or private antigens.
Study design and methods: Since the first documented case of FMAIT due to anti-HPA-9bw (Max(a)), no additional cases have been reported. Here a retrospective analysis is presented of the cases referred to our laboratories in recent years. The diagnosis was performed by genotyping and identification of the maternal alloantibody by the monoclonal antibody-specific immobilization of PLT antigens (MAIPA) technique.
Results: Parental genotyping showed HPA-9bw (Max(a)) mismatch as the sole antigenic incompatibility in seven of eight families. Because the father was found to be HPA-9bw (Max(a)) heterozygous in all the cases, the infant or fetus was genotyped to ascertain the diagnosis. The maternal alloantibody was identified in the MAIPA technique. These data strongly suggest, however, that recognition of the HPA-9bw (Max(a)) epitope is not uniform. The neonatal thrombocytopenia was severe in most cases with bleeding. The outcome was good in all the cases but one.
Conclusion: This analysis confirms that anti-HPA-9bw (Max(a)) FMAIT is not uncommon and was found to be approximately 2 percent of our confirmed FMAIT cases. It is a clinically severe syndrome that requires prompt diagnosis, albeit difficult, and maternal PLT transfusion therapy. Laboratory investigation of a suspected FMAIT case should be carried out in a specialist laboratory well-experienced in optimal testing. Appropriate management and antenatal therapy should be considered for successive pregnancies to prevent fetal bleeding.